"The NTSB said that its ‘testing
found that a spurious high-frequency modulated signal was being created by
parasitic oscillation from the power output transistors in the track circuit
module transmitter. This spurious signal propagated through the power
transistor heat sink, through the metal rack structure, and through a shared
power source into the associated module receiver, thus establishing an
unintended signal path. The spurious signal mimicked a valid track circuit
signal.'

" In separate
letters to Metro, the FTA,
the FRA and Alstom Signaling, Inc., the NTSB revealed that the track circuit
equipment generated a false signal, which ‘resulted in the train not being detected
when it stopped in the track circuit where the accident occurred.' The NTSB
stated that the ‘signal mimicked a valid track circuit signal' and as a result
the ‘ability of the track circuit to detect the train was lost.'

"The NTSB's
recommendations could impact transit systems across the nation. The NTSB stated
that it was ‘concerned about the safety of train control system circuitry used
in comparable rail and transit operations in other parts of the country.'

"The NTSB has identified
a symptom of the problem with the track circuit, but not a root cause or a
solution," said Metro General Manager John Catoe. "We are doing everything we
can to make our rail system as safe as possible. The NTSB still has not
identified a probable cause for the accident. They made two recommendations and
we will implement those recommendations. In fact, we began to do so weeks ago."

"The NTSB recommended
that Metro examine its track circuits that may be susceptible to sending false
signals to eliminate the problem and that Metro develop a program to
periodically determine that electronic components in its train control system
are performing within tolerances. (The NTSB issued similar recommendations to
the Federal Transit Administration and the Federal Railroad Administration,
which oversees transit agencies and railroads in the country that use similar
track circuits.)

"Metro has already begun
to implement the first recommendation by taking the following actions:

1. Metrorail trains are
being operated manually by train operators and will continue in manual mode
until further notice.

2. Metro began addressing
this recommendation the week of the accident when it started running
twice-daily computerized tests of all of its track circuits, once after each
rush hour, or 14 times per week. This computerized analysis identifies similar
events so that track maintenance personnel can make any needed adjustments to
the track circuits. If the analysis identifies a problem, the area of track is
operated in "absolute block," which means only one train at a time is permitted
to be in a designated area of the tracks.

3. Metro also began
addressing this recommendation when it began work with a contractor, ARINC, and
other outside vendors to develop a real-time monitoring system that would
detect circuit failures and generate alerts.

"The second NTSB
recommendation was to ‘develop a program to periodically determine that
electronic components in your train control system are performing within design
tolerances.' Metro also is addressing this recommendation by working with
Alstom Signaling, Inc., the manufacturer of the track signals that exhibited
the failure, and with another signal manufacturer, US&S, to eliminate the
problem identified.

"The NTSB investigation is
not complete," Catoe said. "We will continue to cooperate with the NTSB and
respond quickly in hope that they can identify a root cause or causes that will
allow us to put steps in place to prevent this from happening again."